Ayoub was killed near Aleppo in northern Syria, according to the officials, who spoke on condition of anonymity to discuss the case. On Wednesday, the media arm of Hezbollah posted a picture of Ayoub in military garb as he prayed.

Hezbollah leader Hasan Nasrallah has called the deployment of his fighters to Syria a “new phase” for the movement, and it marks the first time the group has sent significant numbers of men outside Lebanon’s borders.

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May 28, 2014

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Voting begins in Syrian presidential election

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Syrians abroad rushed to cast absentee ballots for the June 3 election that is expected to favor President Bashar al-Assad.

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Syrians abroad rushed to cast absentee ballots for the June 3 election that is expected to favor President Bashar al-Assad.

June 3, 2014 Supporters of Syria's President Bashar al-Assad celebrate in front of a polling center in Damascus. Syrians voted on Tuesday in an election expected to deliver an overwhelming victory for al-Assad but which his opponents have dismissed as a charade in the midst of Syria's devastating civil war.Khaled Al-Hariri/Reuters

Ayoub was part of Hezbollah’s secretive Unit 1800, which has been involved in carrying out terrorist attacks, according Matthew Levitt of the Washington Institute for Near East Policy, who has studied the group.

Levitt, in testimony to Congress, said that Ayoub was convicted by a Romanian court for his role in a Hezbollah plot to hijack an Iraqi airliner in the mid-1980s. He was released from prison in 1988 and then emigrated to Canada.

Ayoub was indicted in Michigan in 2009 on a single charge of using a counterfeit U.S. passport to carry out an attack in Israel on behalf of Hezbollah.

The indictment was unsealed in 2011. Levitt said Ayoub married a U.S. citizen and lived in Michigan at some point.

Ayoub was arrested in 2002 after entering Israel, where he scouted possible targets but was released later as part of a prisoner exchange.

Separately, the FBI is also trying to determine whether a U.S. citizen died in a recent suicide attack in Syria.

U.S. officials said they were aware the man had traveled abroad but could not confirm whether he had died in the attack, which was publicized on Twitter.

Dozens of Americans have traveled to Syria to fight, and some have died. About a dozen with fighting experience have returned to the United States, a senior law enforcement official said.